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Study suggests that decarbonizing US transport sector by converting waste CO2 to fuels would require economical air-capture of CO2

Although technologies for CO2 Capture to produce Transportation Fuels (CCTF)—referring to systems that use sunlight and captured CO2 to make transportation fuels—have the ability to improve domestic energy security, a significant long-term role for CCTF in decarbonizing the US transportation sector (assuming decarbonization of the power generation sector) is “difficult to see” absent the economical capture of CO2 from air, according to a paper by Dr. Tom Kreutz at the Princeton Environmental Institute, Princeton University.

Kreutz used two examples of CCTF systems in his analysis: biodiesel from microalgae and Sandia National Laboratory’s S2P process (an effort to utilize concentrated solar energy to convert waste CO2 into synthetic fuels, earlier post). His paper, Kreutz noted, is only a preliminary scoping study designed to sketch out the rough outlines of each system’s prospective performance and economics as related primarily to GHG emissions.

Kreutz presented the paper at the 10th International Conference on Greenhouse Gas Control Technologies (GHGT-10) earlier this fall in The Netherlands.

Kreutz used what he called a bifurcated climate regime—i.e., pre- and post- decarbonization of the electric power sector—to which he referred as pre-CCS and post-CCS, respectively (although decarbonization was not necessarily via CCS—carbon capture and storage).) In the near-term pre-CCS era, with a low cost of carbon, the economical solution for power providers is to vent the CO2 and pay the fees, passing on the costs to customers.

Over time, however, as the CO2 price increases, it eventually becomes more economical to either retrofit plants to capture and store most of their CO2 (e.g. ~90%) or to “repower” using lower carbon feedstocks or generation technologies (e.g. natural gas, nuclear energy, renewable energy, etc.)...In the pre-CCS regime, CCTF plants must be co-located with (but not necessarily owned by) a power plant; careful integration between the two plants will reduce costs (not studied in detail here).

In the post-CCS regime, fossil-based power plants either employ CCS or have been replaced by nuclear power and/or renewable generators; as a result, large point sources of vented fossil CO2 are relatively rare...In short, large supplies of CO2 are expected to be widely available only as supercritical CO2 in pipelines destined for geologic storage. The amount of pipeline CO2 available for CCTF will depend upon the economic competition (not studied here) between fossil plants with CCS (or CCTF) and non-carbon generators; for simplicity, we assume here that the former is competitive, and pipeline CO2 is plentiful.

—Kreutz (2010)

In CCTF, the source of CO2 determines the net carbon intensity of the fuel, Kreutz says. Direct capture of CO2 from air, or from an exhaust stream vented to the atmosphere, represents negative emissions. When that carbon is converted into a fuel, burned, and exhausted to the atmosphere, the overall cycle is roughly neutral.

However, in the post-CCS regime, if CCTF employs captured CO2 from a pipeline destined for geologic storage (i.e. displaces CCS), the GHG emissions from the combusted synfuel should be comparable to those of traditional petroleum-based fuels, i.e. minimal climate benefit. (Note that the climate benefit is independent of origin of the carbon, e.g. fossil fuels vs. biomass.) It is worth reiterating that, if CCTF employs CO2 captured directly from the air, the resulting fuels are roughly climate neutral. However, because the concentration of atmospheric CO2 is very low, direct air capture is widely believed to be quite costly, and is therefore not considered quantitatively in this study.

—Kreutz (2010)

Among the other conclusions of the study are:

  • In their most economical configuration (without CO2 buffer storage), the low carbon utilization (13-25%) of CCTF severely limits the fraction of US transportation fuels that can be supplied by CCTF, but the carbon utilization can be roughly tripled at modest cost.

  • CCTF most readily provides a significant climate benefit when coupled with large, point source emitters of CO2 that are actively harming the atmosphere, but these are expected to be “scarce resources” in the post-CCS era.

  • CCTF may have an important interim role to play for climate mitigation under a steadily increasing CO2 until the power sector becomes decarbonized, especially if widespread decarbonization is significantly delayed. (This raises the unusual possibility of the transportation sector becoming decarbonized before the power sector, Kreutz notes.) However, after decarbonization, CCTF has the potential to hinder climate mitigation efforts by providing a ready source of only mildly decarbonized domestic transportation fuels. CCTF will only employ direct CO2 capture from air when the CO2 emission price exceeds the cost of air capture.

At sufficiently high oil prices, CCTF will always displace CCS, but from a climate perspective, CCTF (without air capture) is clearly not a replacement for CCS. “Using the carbon twice” fails to meet the objective of deep GHG emission reductions across the entire energy economy; only one sector (either power or transportation)—but not both—can claim the benefit of carbon neutrality. Alternative CCR [CO2 capture and recycle – or reuse]schemes like CCBF [CO2 Capture to produce Boiler Fuel] where carbon is captured and recycled many times, can produce very low carbon energy, but unfortunately not convenient hydrocarbon transportation fuels whose inherently distributed GHG emissions can only be economically mitigated by systems—natural (e.g. biomass) or man-made—that reverse the process by re-capturing CO2 from the atmosphere.

—Kreutz (2010)




One day, we will have to address the problem at the source. Let us not try to capture CO2 and other GHG but let us implement ways to stop their emission.

Clean electric energy generation exist, let us use it.

Clean running vehicles exist, let us use them.

If we do, we would not have to spend $$$B trying to capture and eliminate GHG.

Roger Pham

Exactly! That's why we will need the Hydrogen economy in order to fully utilize renewable energy cost-effectively and efficiently.


Yes hydrogen has its advantages, a FCV can carry more energy per weight than a BEV, but also some disadvantages.

From what I've read, once you take losses due to fuel production, transportation, and storage into account with the tank-to-wheel efficiency you find fuel cell vehicles running on compressed hydrogen may have a power-plant-to-wheel efficiency of just 22% - if the hydrogen is stored as high-pressure gas, and only 17% if it is stored as liquid hydrogen. we may need the Hydrogen economy in order to fully utilize renewable energy but the ERoEI is a b!tch.


Let the plants absorb the CO2 from the air, gasify the biomass and return the bio char carbon to the soil. No need for costly processes to remove CO2 from the air, we create enough from coal fired power plants. Use IGCC and pipe the CO2 to empty natural gas wells for late use.


SJC - Todays money grubbers are far too shortsighted to do that.


You can create pure CO2 by oxyfiring charcoal. Both the hydrogen for the next step and the oxygen for making the CO2 could come from water splitting. When converted to a hydrocarbon fuel the combustion products like H20 (water vapour) and CO2 can be re-absorbed by the biosphere and harvested again. The idea is not to get any new carbon from underground but recycle that already 'in the loop'.

The primary cost gets back to that of splitting water by thermal, electrolytic or biological methods. While synthetic hydrocarbons will be expensive they should have the energy density needed for aircraft fuel and PHEV range extenders

Roger Pham

@ai vin,
What you've read have been old info. H2's is the most efficient synthetic chemical fuel for storage of renewable energy.
Solar and wind electricity is carried via the grid directly to point of local storage and use. At 75-80% efficiency, electrolysis is quite an efficient and very simple way to produce H2 right at the point of retail distribution and/or end use, eliminating other less efficient methods of H2 transportation. High-pressure electrolysis can produce H2 at pressures at 200 bars or above, eliminating another step of less-efficient mechanical compression of the H2. The compressed H2 contains mechanical energy that can be harnessed at the point of use via expanders (like compressed-air energy storage method, but compressed H2 contains 20 times higher energy density!).

I predict that the day will come when the price of PV panels will drop >10 folds , like the prices of big flat panel HDTV's recently, from $14,000 to $1,400 in less than a decade! When this will occur, we can expect solar panels on every roof and every car ports, and we have a huge surplus of solar energy in many seasons, at least in falls and springs. Then, cities will erect local hydrogen depots to storage this energy excess to be used in the winter or in rainy days. Whether you will drive a BEV or a FCV, you will likely benefit from this H2-energy storage. We can gradually wean ourselves fossil fuels forever, and create tens of millions of domestic jobs in the process.

Battery electricity cannot store massive amount nor long-term storage of renewable energy produced in seasons of energy excess, for use in season(s) of high-energy demand.

Henry Gibson

Sodium and sulphur probably represent the cheapest most efficient way to store electrical energy for later use where hydroelectric pumped storage is not available.

Actually the cheapest war to store electrical energy in the US is to not burn coal or natural gas when the grid has wind energy in excess. Computer Automatic controls eliminate the need for large steam turbines which can be replaced with large piston engines that can be started rapidly and are more efficient. Gassified coal can be used when and where natural gas is expensive.

The quickest and a profitable way of reducing energy use is to require combined, heating, power and cooling systems in buildings where natural gas is available, and to do this until there are no gas fired central power plants.

Automobiles can be made that capture their own CO2. ..HG..


Thank you Roger for the update.

BTW what you said about storing energy for the winter reminded me of something: A couple in Maine built a solar house and they found they get MORE solar energy in winter; because on cold days the sky is clearer and the air is drier.

That might not be true at every location but it does suggest that the commonly held "truths" about renewable energy should get a second look.


I wonder if wind power generators could be configured to capture carbon from the atmosphere?


Well joe back in the day I saw a design for a wind turbine that was suppose to get around the problem of having a heavy generator and gearbox way up in the nacelle. The idea was the blades would be open at their tips, as they spun the air inside them would be flung out, creating a vacuum, the air would be replaced by a flow of fresh air through the inside of the turbine and its tower from the base of the tower, where there would be a smaller turbine tapping the internal air flow for power.

Naturally this design never went anywhere. But maybe you could take this design and put a semi-permeable membrane; in the base to sieve out the CO2.

Nah, that's silly idea. lol


There was a project to capture CO2 with membranes like artificial trees, the last I heard it was making some progress. That structure could be put at the base of wind turbine towers, but again there seem to be better ways like letting the plants and farm crops absorb it and return it to the soil.


One way to capture CO2 with wind turbines is use (any) energy to split chalk [ Ca(CO3)2 and water ] into CO2 and lime [Ca(OH)2 ] and spray the lime in the air. The CO2 can be stored.
The lime will capture free CO2 and return it to chalk again. Actually, spraying the lime in the oceans is much easier, and it will alkalize the ocean, which is good, since it is acidified now.

Any green energy source could be used for that (actually concentrated solar heat would be extremely suitable), and the amount of chalk easily available on earth is manyfold the total CO2 in the atmosphere.

This 'chemical reforming proces' of chalk to lime is exactly what is done in a cement plant.

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